The present invention relates generally to air dryers, and more specifically, to an air dryer for a brake system for vehicles.
Air dryers employing membranes consisting of a permeable membrane capable of blocking the passage of nitrogen and oxygen molecules, but allowing water vapor molecules to pass through, as described in U.S. Pat. Nos. 6,719,825 and 5,525,143 amongst many others. In one variation of the technology, the air to be dried passes through the center of the membrane element. The membrane permits the passage of the water vapor to the outside of the membrane, but prevents the passage of the air to the outside of the membrane, thereby drying the air. To work effectively, the outside of the membrane must be kept dry and at a lower pressure than the inside, creating the partial pressure differential to drive the water vapor out. This is done by providing some fraction of the dry output air as a counter flow sweep air across the outside of the membrane. The sweep air can be provided by sweep air orifice(s) connecting the dry air in the membrane air dryer outlet to the sweep air chamber surrounding the outside of the membrane. The orifices control the volume of sweep air, typically 10-20% of the dryer capacity and create a pressure drop in the sweep air volume. The sweep air and entrapped moisture are expelled to atmosphere. U.S. Pat. No. 5,375,620 shows a self-adjusting flow metering device for the sweep air.